1. Technical Field
The present disclosure relates to a switching power supply device in which a primary side and a secondary side are connected by a transformer, and a synchronous rectifier integrated circuit (IC) is used on the secondary side.
2. Description of Related Art
The control of an output voltage obtained on the second side of a switching power supply device has been carried out in the following way, and is shown in JP-A-2012-120399.
FIG. 6 is an example showing a configuration of a switching power supply device using a known secondary side synchronous rectification system in which an AC-DC converter obtains a desired DC voltage from an AC voltage that is rectified by a diode bridge 110, a metal-oxide-semiconductor field-effect transistor (MOSFET), which acts as a switching element, and a transformer. The switching power supply device of FIG. 6 is a simplified version of FIG. 1 in JP-A-2012-120399.
Output voltage information is fed back to a primary side switching control IC 130 via a shunt regulator 210 and a photodiode 220 of a photocoupler, which are disposed on the secondary side of a transformer 200. The shunt regulator 210 controls a current flowing through the shunt regulator 210 using a voltage applied to a reference terminal 212. Herein, the voltage applied to the reference terminal 212 is Vdet whereas output voltage VOUT is divided by resistances R1 and R2.
The control IC 130 controls the switching of a MOSFET 120 disposed on the primary side, and keeps the output voltage VOUT obtained on the secondary side constant.
In the control IC 130, a current flowing to the MOSFET 120 is detected from the voltage of the IS terminal, a feedback voltage from the secondary side is detected at the FB terminal, and the current and the feedback voltage are compared, thereby determining the on-width when switching the MOSFET 120.
To improve the conversion efficiency of the switching power supply device, a MOSFET 240 acts as a synchronous rectification switching element, rather than a diode. MOSFET 240 is applied as a secondary side rectification element, thereby realizing the secondary side synchronous rectification system.
In FIG. 6, with regard to the voltage drop of a rectification element, the conversion efficiency of the switching power supply device is improved considering that Vds (a drain-source voltage) of the MOSFET 240, when turned on, can be lower than Vf (a forward voltage) of a rectification diode, thereby reducing switching loss when under heavy load.
The secondary side synchronous rectification system, having a synchronous rectifier IC 230 disposed on the secondary side is also shown in FIG. 6. Synchronous rectifier IC 230 controls the MOSFET 240 on/off, but it has no function of stopping the switching power supply device when an anomaly occurs. The reason is that even though the synchronous rectifier IC 230 turns off the secondary side MOSFET 240, a primary side switching element continues to turn on/off, and thus the switching power supply device does not stop. What is even worse, a situation can arise in which the synchronous rectifier IC 230 turns off the secondary side MOSFET 240 causing the loss on the secondary side to become larger, and thus generating abnormal heat.
At this point, when the secondary side MOSFET 240 is turned off, a secondary side current continues to flow via a parasitic diode 244 between the drain and the source. When Vf (a forward voltage) of the parasitic diode 244 is larger than Vds (the drain-source voltage) of the MOSFET 240, the switching loss becomes even larger by turning off the MOSFET 240.